In the technology of field emission structures and devices, a microelectronic emission element, or a plurality (array) of such elements, is employed to emit a flux of electrons from one or more field emitters. The field emitter, which often is referred to as a "tip", is specifically shaped to facilitate effective emission of electrons, and may for example be conical-, pyramidal-, or ridge-shaped in surface profile.
Field emitter structures have wide potential and actual utility in microelectronics applications, including electron guns, display devices comprising the field emitter structure in combination with photoluminescent material on which the emitted electrons are selectively impinged, and vacuum integrated circuits comprising assemblies of emitter tips coupled with associated control electrodes.
In typical prior art devices, a field emission tip is characteristically arranged in electrical contact with an emitter conductor and in spaced relationship to an extraction electrode, thereby forming an electron emission gap. With a voltage imposed between the emitter tip and extraction electrode, the field emitter tip discharges a flux of electrons. The tip or tip array may be formed on a suitable substrate such as silicon or other semiconductor material, and associated electrodes may be formed on and/or in the substrate by well-known planar techniques to yield practical microelectronic devices.
Two general field emitter types are known in the art, horizontal and vertical, the direction of electron beam emission relative to the substrate determining the orientational type. Horizontal field emitters utilize horizontally arranged emitters and electrodes to generate electron beam emission parallel to the (horizontally aligned) substrate. Correspondingly, vertical field emitters employ vertically arranged emitters and electrodes to generate electron beam emission perpendicular to the substrate.
Examples of horizontal field emitters are disclosed in Lambe U.S. Pat. No. 4,728,851 and Lee et al U.S. Pat. No. 4,827,177. The Lambe and Lee et al structures are formed as a single horizontal layer on a substrate. An improved horizontal field emitter is disclosed in Jones et al U.S. Pat. No. 5,144,191.
Examples of vertical field emitters are disclosed in Levine U.S. Pat. No. 3,921,022; Smith et al U.S. Pat. No. 3,970,887; Fukase et al. U.S. Pat. No. 3,998,678; Yuito et al U.S. Pat. No. 4,008,412; Hoeberechts U.S. Pat. No. 4,095,133; Shelton U.S. Pat. No. 4,163,949; Gray et al. U.S. Pat. No. 4,307,507; Greene et al U.S. Pat. No. 4,513,308; Gray et al U.S. Pat. No. 4,578,614; Christensen U.S. Pat. No. 4,663,559; Brodie U.S. Pat. No. 4,721,885; Baptist et al U.S. Pat. No. 4,835,438; Borel et al U.S. Pat. No. 4,940,916; Gray et al. U.S. Pat. 4,964,946; Simms et al. U.S. Pat. 4,990,766; and Gray U.S. Pat. No. 5,030,895.
As further examples, Tomii et al U.S. Pat. No. 5,053,673 discloses the fabrication of vertical field emission structures by forming elongate parallel layers of cathode material on a substrate, followed by attachment of a second substrate so that the cathode material layers are sandwiched therebetween in a block matrix. Alternatively, the cathode material layer can be encased in a layer of electrically insulative material sandwiched in such type of block matrix. The block then is sectioned to form elements having exposed cathode material on at least one face thereof. In the embodiment wherein the cathode material is encased in an insulative material, the sliced members may be processed so that the cathode material protrudes above the insulator casing. The exposed cathode material in either embodiment then is shaped into emitter tips (microtip cathodes).
Spindt et al U.S. Pat. No. 3,665,241 discloses vertical field emission cathode/field ionizer structures in which "needle-like" elements such as conical or pyramidal tips are formed on a (typically conductive or semiconductive) substrate. Above this tip array, a foraminous electrode member, such as a screen or mesh, is arranged with its openings vertically aligned with associated tip elements. In one embodiment disclosed in the patent, the needle-like elements comprise a cylindrical lower pedestal section and an upper conical extremity, wherein the pedestal section has a higher resistivity than either the foraminous electrode or the upper conical extremity, and an insulator may be arranged between the conical tip electrodes and the foraminous electrode member. The structures of this patent may be formed by metal deposition through a foraminous member (which may be left in place as a counter-electrode, or replaced with another foraminous member) to yield a regular array of metal points.
A metal microtip process conventionally employed in the art to fabricate structures of the type disclosed in the Spindt et al. patent involves the initial fabrication of a basic structure on a substrate of a material such as glass, on which are successively deposited cathode, insulator and gate material layers. The uppermost gate material layer is photomasked, and RIE processed to form an opening in the gate material layer, exposing the underlying insulator layer. The underlying layer of insulator material, e.g., SiO2, is then etched by chemical etch or RIE technique, to yield a cavity below the gate layer opening and extending down to the cathode material layer. This cavity extends radially outwardly under the overlying gate layer, so that the latter forms an overhang over the cavity about its periphery.
Subsequently in this microtip emitter structure formation process, a parting layer is vacuum deposited on the gate layer by evaporation technique, at a shallow angle (e.g., along a direction which is 75 degrees from the central axis of the cavity). The microtip element then is formed in the cavity on the cathode layer with contemporaneous formation of a closure layer overlying the parting layer on the gate structure. Finally, the parting layer is electrochemically etched to remove the closure layer, and yield the final structure in which the gate layer forms a gate electrode structure overlyingly surrounding the conical emitter tip in the cavity.
Jones et al U.S. Pat. No. 5,371,431 discloses a vertical column emitter structure in which the columns include a conductive top portion and a resistive bottom portion, and upwardly vertically extend from a horizontal substrate. By this arrangement, an emitter tip surface is provided at the upper extremity of the column and the tip is separated from the substrate by the elongate column. An insulating layer is formed on the substrate between the columns. An emitter electrode may be formed at the base of the column and an extraction electrode may be formed adjacent the top of the column.
As described in Jones et al U.S. Pat. No. 5,371,431, the vertical column emitter structure may be fabricated by forming the tips on the face of the substrate, followed by forming trenches in the substrate around the tips to form columns having the tips at their uppermost extremities. Alternatively, the vertical column emitter structure of U.S. Pat. No. 5,371,431 is described as being fabricatable by forming trenches in the substrate to define columns, followed by forming tips on top of the columns. In either method, the trenches may be filled with a dielectric and a conductor layer may be formed on the dielectric to provide extraction electrodes.
Further improvements in vertical field emitter structures and fabrication methods are disclosed in Jones U.S. patent application Ser. No. 029,880, filed Mar. 11, 1993, entitled "Emitter Tip Structure and Field Emission Device Comprising Same, and Method of Making Same," and in corresponding International Application Number PCT/US94/02669, published on 15 Sep. 1994 as International Publication WO 94/20975.
By the present invention, a number of structures are provided which enhance the performance and reliability of field emitter devices, particularly field emitter displays. The invention additionally provides methods for fabricating the structures.
More particularly, the invention provides various improved structures and methods for readily fabricating arrays of field emitter elements in a base structure, in which the field emitter elements have superior uniformity of shape and dimensional character, and resulting enhanced utility for field emitter displays, as compared to field emitter elements formed by prior art fabrication techniques.
FIGS. 1-3 depict a process for forming a base structure for subsequent fabrication of emitter tip elements thereon.
FIGS. 4-6 depict an alternative process to that shown in FIGS. 1-3 for forming a base structure for subsequent fabrication of emitter tip elements thereon.
FIGS. 7-9 depict the etch formation of emitter tip elements on a base structure of the type formed via the processes of FIGS. 1-3 or FIGS. 4-6.
FIGS. 10-16 depict the evaporation formation of emitter tip elements on a base structure of the type formed via the processes of FIGS. 1-3 or FIGS. 4-6, with FIGS. 10-12 and 15-16 showing schematically the structures in the processs flow and with FIGS. 13 and 14 showing photomicrographs of the "veiled" precursor structure of the field emission array and of the final field emission array structure.